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Module 1: An Overview of Engine Emissions and Air PollutionLecture 1: Introduction to ICEngines and Air Pollution

Historical Overview of IC Engine Development

The modern reciprocating internal combustion engines have their origin in the Otto and Diesel Engines

invented in the later part of 19th century. The main engine components comprising of piston, cylinder,

crank-slider crankshaft, connecting road, valves and valve train, intake and exhaust system remain

functionally overall similar since those in the early engines although great advancements in their design

and materials have taken place during the last 100 years or so. An historical overview of IC engine

development with important milestones since their first production models were built, is presented in

Table 1.1

Table 1.1

Historical Overview and Milestones in IC EngineDevelopment

Year Milestone

1860-

1867

J. E. E. Lenoir and Nikolaus Otto developed atmospheric engine wherein combustion of

fuel-air charge during first half of outward stroke of a free piston accelerating the piston

which was connected to a rack assembly. The free piston would produce work during

second half of the stroke creating vacuum in the cylinder and the atmospheric pressure

then would push back the piston.

1876 Nikolaus Otto developed 4-stroke SI engine where in the fuel-air charge was compressedbefore being ignited.

1878 Dougald Clerk developed the first 2-stroke engine

1882

Atkinson develops an engine having lower expansion stroke than the compression stroke

for improvement in engine thermal efficiency at cost of specific engine power. The Atkinson

cycle is finding application in the modern hybrid electric vehicles (HEV)

1892

Rudolf Diesel takes patent on engine having combustion by direct injection of fuel in the

cylinder air heated solely by compression , the process now known as compression

ignition (CI)

1896 Henry Ford develops first automobile powered by the IC engine

1897 Rudolph Diesel developed CI engine prototype, also called as the Diesel engine

1923Antiknock additive tetra ethyl lead discovered by the General Motors became commercially

available which provided boost to development of high compression ratio SI engines

1957 Felix Wankel developed rotary internal combustion engine

1981 Multipoint port fuel injection introduced on production gasoline cars

1988 Variable valve timing and lift control introduced on gasoline cars


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1989-

1990Electronic fuel injection on heavy duty diesel introduced

1990 Carburettor was replaced by port fuel injection on all US production cars

1994Direct injection stratified charge (DISC) engine powered cars came in production by

Mitsubishi and Toyota

IC Engine Classification based on Combustion Process

IC Engines may be classified based on the state of air-fuel mixture present at the time of ignition in the

engine cycle, the type of ignition employed and the nature of combustion process subsequent to ignition

of the air-fuel mixture.

A. Physical State of Mixtureo Homogeneous Charge

Premixed outside( conventional gasoline and gas engines with fuel inducted in

the intake manifold)

Premixed in-cylinder: In- cylinder direct injection and port fuel injectiono Heterogeneous Charge

B. Ignition Type

o Positive source of Ignition e.g., spark ignitiono Compression ignition

C. Mode of Combustion

o Flame propagation

o Spray combustion

This course primarily deals with combustion generated engine emissions and approaches the subject

from the point of fundamentals of engine combustion processes. The engines are therefore, categorized

based on the mode of ignition employed viz., Spark Ignit io n (SI) Engines and Compression Igni t ion

(CI) Engines.

Method of ignition has been adopted as the main criterion of classification as in the conventional type

IC engines it governs

Fuel type

Mixture preparation methods

Progression of combustion process

Combustion chamber design

Engine load control, and

Operating and emission characteristics

More advanced and newer combustion systems are dealt as special variations of the IC engines. For


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example the direct injection stratified charge (DISC) engine is taken as a special variant of SI engine.

The homogeneous charge compression ignition engines are being developed around the conventional SI

and CI engines and are discussed accordingly.

Main Events in Four-Stroke SI Engine Cycle

Figure 1.1 shows typical pressure crank angle (P-) history for a four-stroke SI engine cycle. The sequence

of main events in the cycle are given in Table 1.2

Figure 1.1 Sequence of Events in 4-Stroke SI Engine Cycles

Table 1.2

Sequence of Events in 4-Stroke SI EngineCycle

Event Time of Occurrence, Crank angle


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Intake valve opens (IO) 20 - 5 CA bTDC at the end of exhaust stroke

Exhaust valve closes (EC) 8 to 20 CA aTDC in the beginning of intake stroke

Intake valve closes (IC) 60 -40 CA aBDC in the beginning of compressionstroke

Spark ignition45 -15 CA bTDC towards the end of compression

stroke

Combustion by turbulent

flame propagation

Begins shortly after ignition up to 15 to 30 CA aTDC

Early in the expansion stroke

Exhaust valve opens (EC)50 -30 CA bBDC Shortly before the end of expansion

stroke

CA: Crank Angle, ATDC: After Top Dead Centre; BTDC: Before Top Dead Centre; ABDC: After Bottom

Dead Centre;

BBDC:Before Bottom Dead Centre;

Main Events in Four-Stroke CI Engine Cycle

Figure 1.2 shows typical pressure crank angle (P-) history for a four-stroke CI engine cycle. The

sequence of main events in the cycle are given in Table 1.3


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Figure 1.2 Main Events in Four-Stroke CI Engine Cycle

Table 1.3Sequence of Events in 4-Stroke CI Engine

Cycle

Event Time of occurrence, Crank angle

Intake valve opens

(IO)5 -20 CA bTDC at the end of exhaust stroke

Exhaust valve

closes (EC)8 to 20 CA aTDC in the beginning of intake stroke

Intake valve closes

(IC)40 -20 CA aBDC in the beginning of compression stroke

Start of Injection

(SOI)

15-5 CA bTDC towards the end of compression stroke. Injection duration

at full engine load about 15 to 25 CA

Start of combustion

(SOC)5 -0 CA bTDC, (considering ignition delay after injection)

End of combustion 20 to 30 CA aTDC in expansion stroke


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(EOC)

Exhaust valve opens

(EC)40 to 30 CA bBDC Shortly before the end of expansion stroke

Lecture 2: Engine Emissions and Air Pollution

Principal Engine Emissions

SI Engines CO, HC and NOx

CI Engines CO, HC, NOx and PM

CO = Carbon monoxide, HC = Unburned hydrocarbons, NO x = Nitrogen oxides mainly mixture of NO

and NO2 ,

PM = Particulate matter

Other engine emissions include aldehydes such as formaldehyde and acetaldehyde primarily from the

alcohol fuelled engines, benzene and polyaromatic hydrocarbons (PAH).

Sources of Engine/Vehicle Emissions

Figure 1.3 shows the sources of emissions from a gasoline fuelled SI engine viz., exhaust, crankcase

blow by and fuel evaporation from fuel tank and fuel system


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Figure. 1.3 Emission sources in a gasoline fuelled carFrom a diesel engine powered vehicle the emission sources are shown in Fig. 1.4.

Figure 1.4 Emission sources in a diesel engine powered bus.

Emissions and Pollutants


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Engine emissions undergo chemical reactions in atmosphere known largely as photochemical

reactions and give rise to other chemical species which are hazardous to health and environment.

Linkage of engine emissions and air pollutants is shown in Fig. 1.5.

TSP = Total suspended particulate matter in airPAN = Peroxy- acetyl nitrate

Figure. 1.5 Air pollutants resulting from engine emissions

Photochemical Smog

Photochemical smog is a brownish-gray haze resulting from the reactions caused by solar

ultraviolet radiations between hydrocarbons and oxides of nitrogen in the atmosphere. The air

pollutants such as ozone, nitric acid, organic compounds like peroxy- acetylnitrates or PAN (

CH3CO-OO-NO2) are trapped near the ground by temperature inversion experienced especially

during winter months. These chemical substances can effect human health and cause damage to

plants. The photochemical reactions are initiated by nitrogen oxides emitted by vehicles into

atmosphere. A simple set of reactions leading to photochemical smog formation is as follows:


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is energy of a photon and UV is ultraviolet light radiations .

he above reactions form NO2 photolytic cycle. However, if only these reactions are involved then,NO2concentration in the atmosphere would remain constant. But, volatile organic compounds (VOCs) thatinclude unburned hydrocarbons and their volatile derivatives also react with NO and O2 to form NO2 . Thereactions between HC and NO do not necessarily involve ozone and provide another route to formNO2 and thus, the concentration of ozone and NO2 in the urban air rises. The most reactive VOCs inatmosphere are olefins i.e., the hydrocarbons with C=C bond. The general reaction between

hydrocarbons (RH) and NO may be written as

The overall global reaction is

Main processes in photochemical smog formation are shown in Fig. 1.6.


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Figure1.6 Main processes in photochemical smog formation (adaptedfrom http://mtsu32.mtsu.edu:11233/Smog-Atm1.htm)

The harmful constituents of photochemical smog are, NO2, O3, PAN and aldehydes. The PAN andaldehydes cause eye irritation. NO2 and ozone are strong oxidants and cause damage to elastomeric/

rubber materials and plants.

Photochemical Reactivity of Hydrocarbons

The exhaust gases of gasoline engines contain more than 150 different hydrocarbons and theirderivatives. Some hydrocarbons are more reactive than the others. The photochemical reactivity ofhydrocarbons has been measured in terms of the rate at which the specific hydrocarbon causesoxidation of NO to NO2. To determine the rate of photo-oxidation, NO in presence of the specifichydrocarbon is irradiated by ultra violet radiations in a reaction chamber and the buildup of NO2 in termsparts per billion/per minute is recorded. Another photochemical reactivity scale has been defined in terms

of ozone formation. Reactivity of different classes of hydrocarbons based on formation of NO2 is given inTable 1.4It has been noted that the reactivity of a given hydrocarbon depends also on the initial concentrations ofpollutants in the environment in which a particular hydrocarbon is added when emitted. A reactivity

termed as incr emental activi tyhas been determined in terms of ozone formed. It is defined as the

change in ozone formation rate when specific VOC is added to the base reactive organic gas mixture inthe environment divided by the amount of the specific VOC added. This reactivity is considered to bge ofmore practical relevance.


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Table 1.4

Photochemical Reactivity of Hydrocarbons(General Motor Scale)

Hydrocarbon RelativeReactivity*

C1-C4 paraffins AcetyleneBenzene 0

C4 and higher paraffins Monoalkyl benzenes Ortho- andpara-

dialkyl benzenes Cyclic paraffins2

Ethylene Meta- dialkyl benzenes Aldehydes 7

1-olefins (except ethylene) Diolefins Tri- and tetraalkyl benzenes 10

Internally bonded olefins 30

Internally bonded olefins with substitution at double bondCyclo-

olefins100

*based on NO2 formation rate for the specific hydrocarbon relative to that for 2,3 dimethyl-2-benzene

Health Effects of Air Pollutants

The effect of pollutants on human health depends on pollutant concentration in the ambient air and the

duration to which the human beings are exposed. Adverse health effects of different pollutants on human

health are given in Table 1.5 for short term and long term exposures. Carbon monoxide on inhalation is

known to combine with haemoglobin at a rate 200 to 240 times faster than oxygen thus reducing

oxygen supply to body tissues and results in CO intoxication. Nitrogen oxides get dissolved in mucous

forming nitrous and nitric acids causing irritation of nose throat and respiratory tract. Long term exposure

causes nitrogen oxides to combine with haemoglobin and destruction of red blood cells. Long term

exposure resulting in more than 10% of haemoglobin to combine with nitrogen oxides causes bluish

colouration of skin, lips fingers etc

Table 1.5

Adverse Health Effects of IC EngineGenerated Air Pollutants


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Pollutants Short-term healtheffects Long-term health effects

Carbon monoxide

Headache, shortness of

breath, dizziness, impaired

judgment, lack of motor

coordination

Effects on brain and central nervous system,

nausea, vomiting, cardiac and pulmonary

functional changes, loss of consciousness

and death

Nitrogen dioxideSoreness, coughing, chest

discomfort, eye irritation

Development of cyanosis especially at lips,

fingers and toes, adverse changes in cell

structure of lung wall

OxidantsDifficulty in breathing, chest

tightness, eye irritation

Impaired lung function, increased

susceptibility to respiratory function

OzoneSimilar to those of NO2 but at a

lower concentration

Development of emphysema, pulmonary

edema

Sulfates Increased asthma attacksReduced lung function when oxidants are

present

TSP/Respirable

suspended

particulate

Increased susceptibility to

other pollutants

Many constituents especially poly-organic

matter are toxic and carcinogenic, contribute

to silicosis, brown lung

Historical Overview: Engine and Vehicle Emission Control

Beginning with the identification during early 1950s that mainly the unburned hydrocarbons and nitrogen

oxides emitted by vehicles are responsible for formation of photochemical smog in Los-Angeles region in

the US, the initiatives and milestones in pursuit of vehicle/ engine emission control are given in Table 1.6

Table 1.6

Engine Emission Control A HistoricalPerspective

Year Event and Milestone

1952Prof A. J. Haagen- Smit of Univ. of California demonstrated that the photochemicalreactions between unburned hydrocarbons (HC) and nitrogen oxides (NOx) are

responsible for smog (brown haze) observed in Los- Angeles basin

1965 The first vehicle exhaust emissions standards were set in California, USA

1968 The exhaust emission standards set for the first time throughout the USA

1970 Vehicle emission standards set in European countries


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1974

Exhaust catalytic converters for oxidation of carbon monoxide (CO) and HC were

needed in the US for meeting emission targets. Phasing-out of tetra ethyl lead (TEL),

the antiknock additive from gasoline begins to ensure acceptable life of the catalytic

converters

1981Three-way catalytic converters and closed-loop feedback air-fuel ratio control for

simultaneous conversion of CO, HC and NOx introduced on production cars

1992Euro 1 emission standards needing catalytic emission control on gasoline vehicles

implemented in Europe

1994 Catalytic emission control for engines under lean mixture operation introduced

1994US Tier -1 standards needing reduction in CO by nearly 96%, HC by 97.5% and NOx by

90%

2000-

2005

Widespread use of diesel particulate filters and lean de-NOx catalyst systems on heavy

duty vehicles

2004 US Tier -2 standards needing reduction in CO by nearly 98 %, HC by 99% and NOx by95%

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